1. Field of the Invention
The present invention relates to an air-fuel ratio control device for an engine that controls an air-fuel ratio of the engine based on at least an output of an air-fuel ratio sensor disposed in the exhaust passage upstream of a three-way catalyst. More specifically, the present invention relates to an air-fuel control device that is able to detect deterioration of the three-way catalyst based on output signals of air-fuel ratio sensors disposed in the exhaust passage upstream and downstream of the three-way catalyst even during a transition of the operating condition of the engine.
2. Description of the Related Art
An air-fuel ratio control device for controlling the air-fuel ratio of an engine by feedback control based on an output of one air-fuel ratio sensor (such as an O.sub.2 sensor) disposed in an exhaust passage upstream of a catalytic converter is known as a single O.sub.2 sensor system. The single O.sub.2 sensor system is used to control the air-fuel ratio of the engine at a stoichiometric air-fuel ratio to thereby improve the condition of exhaust emissions by utilizing the ability of the three-way catalytic converter to a maximum degree.
Also, to compensate for the individual difference among cylinders or changes due to aging of the upstream O.sub.2 sensor, a double O.sub.2 sensor system using two O.sub.2 sensors has been developed(U.S. Pat. No. 4,739,614). In the double O.sub.2 sensor system, O.sub.2 sensors are disposed upstream and downstream of the catalytic converter in the exhaust passage, and the air-fuel ratio control is carried out based on the output of the downstream O.sub.2 sensor as well as the output of the upstream O.sub.2 sensor.
Nevertheless, even in the double O.sub.2 sensor system, if the catalyst in the catalyst converter deteriorates, the condition of the exhaust emissions such as HC, CO, NO.sub.x deteriorates. Therefore, it is necessary to detect the deterioration of the catalyst accurately.
To detect the deterioration of the catalyst in the catalytic converter, various methods and devices have been proposed.
For example, the deterioration of the catalyst can be determined by detecting a reduction in the O.sub.2 storage capacity of the catalyst. That is, the catalyst has an ability to adsorb oxygen from the exhaust gas when the air-fuel ratio is lean compared to the stoichiometric air-fuel ratio (i.e., the air-fuel ratio of the exhaust gas is higher than the stoichiometric air-fuel ratio), and to release said oxygen when the air-fuel ratio is rich compared to the stoichiometric air-fuel ratio (i.e., the air-fuel ratio of the exhaust gas is lower than the stoichiometric air-fuel ratio). This capacity, i.e., the O.sub.2 storage capacity of the catalyst, falls as the deterioration of the catalyst proceeds. Therefore, when the catalyst is in a normal condition, the fluctuation of the air-fuel ratio of the exhaust gas downstream of the catalytic converter is small, and consequently, the fluctuation of the output of the downstream O.sub.2 sensor also becomes small even if the air-fuel ratio of the exhaust gas oscillates between a rich air-fuel ratio and a lean air-fuel ratio. On the contrary, if the catalyst has deteriorated, the air-fuel ratio of the exhaust gas downstream of the catalytic converter oscillates in a manner similar to the oscillation of the air-fuel ratio of the exhaust gas upstream of the catalytic converter due to the deterioration of the O.sub.2 storage capacity of the catalyst, and the output of the downstream O.sub.2 sensor also fluctuates as the output of the upstream O.sub.2 sensor fluctuates. Therefore, when the catalyst has deteriorated, the interval between reversals of the output signal of the downstream O.sub.2 sensor (i.e., the period between changes of the output signal of the downstream O.sub.2 sensor from a rich air-fuel ratio signal to a lean air-fuel ratio signal, or vice versa) during air-fuel ratio feedback control becomes shorter (in other words, the number of the reversals of the output signal of the downstream O.sub.2 sensor within a predetermined time becomes larger), and the amplitude of the fluctuations in the output signal of the downstream O.sub.2 sensor becomes larger at the same time.
In the system disclosed in U.S. Pat. No. 4,739,614, it is determined that the catalyst has deteriorated when the ratio of the intervals between the reversals of the upstream O.sub.2 sensor T.sub.1 to the intervals between the reversals of the downstream O.sub.2 sensor T.sub.2, i.e., T.sub.1 /T.sub.2 becomes larger than a predetermined value (or, alternatively, when the interval T.sub.2 of the downstream O.sub.2 sensor becomes smaller than a predetermined value).
However, in the above system, if the center value of the air-fuel ratio feedback control deviates from the stoichiometric air-fuel ratio, the reversal of the output of the downstream O.sub.2 sensor does not cross the stoichiometric line, and oscillates in a small amplitude on a rich side or a lean side of the air-fuel ratio. In this case, deterioration of the catalyst cannot be detected by the intervals or the number of the reversals of the output of the downstream O.sub.2 sensor. Also, it is not possible to detect deterioration from the amplitude of the oscillation of the output of the downstream O.sub.2 sensor since the amplitude of the output of the downstream O.sub.2 sensor becomes very small regardless of the deterioration of the catalyst in this case.
Further, if the downstream O.sub.2 sensor itself has deteriorated, the amplitude of the output of the downstream O.sub.2 sensor becomes smaller regardless of the deterioration of the catalyst. Therefore, it is sometimes very difficult to detect deterioration of the catalyst accurately based on the intervals or the number of the reversals as well as the amplitude of the output of the downstream O.sub.2 sensor.
To solve above-mentioned problems, the copending U.S. patent application Ser. No. 957,041 proposes a method for detecting deterioration of the catalyst based on a ratio of the lengths of the output signal response curves of the upstream and downstream O.sub.2 sensors and a ratio of the areas bounded by the output signal response curves and reference lines.
When the catalyst is in normal condition, the air-fuel ratio of the exhaust gas downstream of the catalytic converter fluctuates in a small amplitude about the stoichiometric air-fuel ratio due to the O.sub.2 storage capacity of the catalyst even when the air-fuel ratio of the exhaust gas upstream of the catalytic converter oscillates between a rich side air-fuel ratio and a lean side air-fuel ratio. In this case, the fluctuation of the output of the downstream O.sub.2 sensor is smaller compared to the fluctuation of the output of the upstream O.sub.2 sensor. Therefore, the length LVOS of the output signal response curve of the downstream O.sub.2 sensor becomes smaller compared to the length LVOM of the output signal response curve of the upstream O.sub.2 sensor. Consequently, the ratio of the lengths LVOS/LVOM also becomes small when the catalyst is in normal condition.
On the contrary, when the catalyst has deteriorated, the air-fuel ratio of the exhaust gas downstream of the catalyst converter oscillates in a similar manner as the oscillation of the air-fuel ratio upstream of the catalytic converter due to reduction of the O.sub.2 storage capacity of the catalyst. In this case, the length LVOS of the output signal response curve of the downstream O.sub.2 sensor increases since the amplitude and the frequency of the fluctuation of the output signal of the downstream O.sub.2 sensor increase, though the length LVOM of the output response curve of the upstream O.sub.2 sensor stays same. Therefore, the value of the ratio of the lengths LVOS/LVOM increases, and approaches to 1.0 as the deterioration of the catalyst proceeds. Thus, deterioration of the catalyst can be detected by monitoring the value LVOS/LVOM as far as the O.sub.2 sensors are not deteriorated.
However, if either of the O.sub.2 sensors has deteriorated, deterioration of the catalyst cannot be detected accurately by the value of the ratio LVOS/LVOM any more.
When deteriorated, the amplitude of the output of the O.sub.2 sensor becomes smaller. This causes a decrease in the length of the output signal response curve of the deteriorated O.sub.2 sensor. Therefore, for example, if the downstream O.sub.2 sensor has deteriorated, the length LVOS does not increase largely even if the catalyst has deteriorated. In this case, a deteriorated catalyst may be incorrectly determined as being normal since the increase in the ratio LVOS/LVOM is relatively small.
On the other hand, if the upstream O.sub.2 sensor has deteriorated, the length LVOM becomes smaller regardless of the deterioration of the catalyst. In this case, a normal catalyst may be incorrectly determined as being deteriorated since the ratio LVOS/LVOM increases.
To prevent such an incorrect determination, the method proposed by the copending U.S. application Ser. No. 957,041 uses the ratio of the area AVOS bounded by the output signal response curve of the downstream O.sub.2 sensor and a reference line, and the area AVOM bounded by the output signal response curve of the upstream O.sub.2 sensor for detecting deterioration of the catalyst in addition to the ratio of the lengths LVOS/LVOM.
FIGS. 22 and 23 show the changes in the values of the ratio of the lengths LVOS/LVOM and the ratio of the areas AVOS/AVOM in accordance with the deterioration of the upstream O.sub.2 sensor and downstream O.sub.2 sensor when the catalyst is normal (FIG. 22), and when the catalyst has deteriorated (FIG. 23). In FIGS. 22 and 23, the columns with a .smallcircle. mark indicate waveforms of the output signal response curves of O.sub.2 sensors in normal condition, and the columns with a x mark indicate waveforms of the output signal response curves of deteriorated O.sub.2 sensors.
As seen from FIGS. 22 and 23;
(1) The output of the upstream O.sub.2 sensor is only affected by the deterioration of the upstream O.sub.2 sensor itself. Namely, if the upstream O.sub.2 sensor is not deteriorated, both the length LVOM and the area AVOM are large regardless of the deterioration of the catalyst (FIG. 22, (1), (2) and FIG. 23, (5), (6))). If the upstream O.sub.2 sensor has deteriorated, both the length LVOM and AVOM become small regardless of the deterioration of the catalyst (FIG. 22, (3), (4) and FIG. 23, (7), (8)).
(2) The output of the downstream O.sub.2 sensor is affected by both the deterioration of the downstream O.sub.2 sensor itself and the deterioration of the catalyst.
Namely, If the catalyst is not deteriorated;
1 The length LVOS is small regardless of the deterioration of the downstream O.sub.2 sensor (FIG. 22, (1) through (4)); PA1 2 The area AVOS is large when the downstream O.sub.2 sensor is not deteriorated (FIG. 22, (1) and (3)), and is medium when the downstream O.sub.2 sensor has deteriorated (FIG. 22, (2) and (4)). PA1 1 Both of the length LVOS and the area AVOS are large when the downstream O.sub.2 sensor is not deteriorated (FIG. 23, (5) and (7)); PA1 2 The length LVOS is medium and the area AVOS is small when the downstream O.sub.2 sensor has deteriorated (FIG. 23, (6) and (8)).
And, if the catalyst has deteriorated;
Accordingly, the ratio of the lengths LVOS/LVOM and the ratio of the areas take the values as shown in the right side columns of FIGS. 22 and 23. It will be understood that in the cases of FIG. 22, (3) (the catalyst is normal, the upstream O.sub.2 sensor has deteriorated) and FIG. 23 (6) (both the catalyst and the downstream O.sub.2 sensor have deteriorated), deterioration of the catalyst can not be determined by the ratio LVOS/LVOM only since LVOS/LVOM becomes medium in both cases in spite of the difference of the presence of the catalyst deterioration.
Even in these cases, the value of the ratio AVOS/AVOM is very large when the catalyst is not deteriorated (FIG. 22, (3)) and is small when the catalyst has deteriorated (FIG. 23, (6)). Therefore, by using the ratio AVOS/AVOM in addition to the ratio LVOS/LVOM, the deterioration of the catalyst can be detected even in these cases. In the method proposed by copending application Ser. No. 957,041, deterioration of the catalyst is detected based on the relationship of LVOS/LVOM and AVOS/AVOM explained above, i.e., for example, it is determined that the catalyst has deteriorated when the values LVOS/LVOM and AVOS/AVOM fall in the hatched area in the map shown in FIG. 24.
However, in the method explained above, the degree of the deterioration of the catalyst is not taken into consideration. Since the deterioration of the catalyst proceeds gradually, the catalyst still maintains sufficient capacity for removing some pollutants in the exhaust gas even though deterioration has already started. Therefore, it is necessary to distinguish these catalysts still having sufficient capacity (hereinafter, called "catalysts with a medium level of deterioration") from the catalysts in which deterioration proceeds largely and the capacity for removing pollutants becomes insufficient (hereinafter, called "catalysts with a high level of deterioration").
In the method explained above, it is found difficult to distinguish the catalyst with a medium level of deterioration from the catalyst with a high level of deterioration under some operating condition. This problem is explained with reference to FIGS. 1A through 3B in detail.
FIG. 2 shows examples of changes in the values of the ratios LVOS/LVOM and AVOS/AVOM in accordance with the level of the deterioration of the catalyst (medium level of deterioration or high level of deterioration), the condition of the downstream O.sub.2 sensor (normal or deteriorated), and the operating condition of the engine (stable or transient), but, in all cases shown in FIG. 2, it is assumed that the upstream O.sub.2 sensor is not deteriorated.
Nos. 1, 2, 5, 6 in FIG. 2 show the cases of transient operating condition of the engine such as during acceleration or deceleration, and Nos. 3, 4, 7, 8 show the cases of stable operating condition of the engine such as during constant load and constant speed operation, and the values LVOS/LVOM and AVOS/AVOM of these cases are shown in the right side of corresponding columns in FIG. 2. FIG. 3A indicates values LVOS/LVOM and AVOS/AVOM of cases 1 through 8 of FIG. 2 on the map of FIG. 24 used in U.S. application Ser. No. 957,041.
As seen from FIG. 3A, the values LVOS/LVOM and AVOS/AVOM of the catalyst with a high level of deterioration (cases No. 1 to No.4) are always in the hatched area of the map (i.e., determined as being deteriorated) regardless whether the operating condition of the engine is stable or transient. However, the values of the catalyst with a medium level of deterioration (cases No. 5 to No. 8) fall in different areas on the map depending on whether the operating condition of the engine is stable or transient.
For example, both No. (5) and No. (7) in FIGS. 2 and 3A represent the combination of a catalyst with a medium level of deterioration and a normal downstream O.sub.2 sensor. However, when the operating condition of the engine is stable, the catalyst is determined as being deteriorated (FIG. 3A, No. (7)), though the same catalyst is determined as being normal when the operating condition is transient (FIG. 3A, No. (5)).
Also, in the cases No. (6) and No. (8) in FIGS. 2 and 3A (i.e., the combination of a catalyst with a medium level of deterioration and a deteriorated downstream O.sub.2 sensor), the same catalyst is determined as being deteriorated when the operating condition is stable (FIG. 3A, No. (8)), and determined as being deteriorated when the operating condition is transient (FIG. 3A, No. (6)).
This means that, when the detection of deterioration of the catalyst is carried out in a stable operating condition, the catalyst with a medium level of deterioration, which still maintains practically sufficient capacity for removing pollutants in the exhaust gas, is determined to be in the same condition as the catalyst with a high level of deterioration which must be replaced as soon as possible.
The above difference in the result of the determination occurs because the value of the ratio of areas AVOS/AVOM tends to decrease during the stable operating condition of the engine (in other words, the ratio AVOS/AVOM increases during the transient operating condition) compared to the value during the transient operating condition. The reason why the ratio of the areas decreases during the stable operating condition is explained by referring FIGS. 1A through 1D.
FIGS. 1A through 1D show the waveforms of the output signal of the upstream O.sub.2 sensor (FIGS. 1A and 1C) and the downstream O.sub.2 sensor (FIGS. 1B and 1D) when the catalyst with a medium level of deterioration is used. Further, FIGS. 1A and 1B show the case when the operating condition of the engine is stable, and FIGS. 1C and 1D show the case when the operating condition of the engine is transient.
When the operating condition of the engine is transient, i.e., when the engine speed is accelerated or decelerated, the center value of the air-fuel ratio feedback control tends to deviate from the stoichiometric air-fuel ratio to the lean air-fuel ratio side (during acceleration) or the rich air-fuel ratio side (during deceleration). Consequently, it takes longer to adjust the center value to the stoichiometric air-fuel ratio by air-fuel ratio feedback control during the transition of the operating condition. This causes the period of the air-fuel ratio feedback control (represented by T in FIGS. 1A and 1C) to become longer. Also, the air-fuel ratio of the engine tends to deviate considerably from the stoichiometric air-fuel ratio during the transition of the operating condition (FIG. 1C). In this case also, the period of the fluctuation in the output signal of the O.sub.2 sensor downstream of the catalyst with a medium level of deterioration becomes longer, and the output signal of the downstream O.sub.2 sensor tends to deviate from the reference value corresponding to the stoichiometric air-fuel ratio (FIG. 1D).
On the other hand, when the operating condition of the engine is stable, the center value of the air-fuel ratio feedback control is easily adjusted to the stoichiometric air-fuel ratio and the air-fuel ratio of the engine oscillates regularly between the lean air-fuel ratio and the rich air-fuel ratio. Also, the period of the air-fuel ratio feedback control becomes shorter. In this case, the output signal of the O.sub.2 sensor downstream of the catalyst with a medium level of deterioration fluctuates near the reference value corresponding to the stoichiometric air-fuel ratio, and the period of the fluctuation also becomes small.
In other words, the area AVOS of the output signal response curve of the downstream O.sub.2 sensor in the normal operating condition decreases largely from the area in the transient operating condition (refer to FIGS. 1B and 1D), though the decrease in the area AVOM of the output signal response curve of the upstream O.sub.2 sensor is relatively small (refer to FIGS. 1A and 1C).
This causes the ratio of the area AVOS/AVOM to become smaller in the stable operating condition than in the transient operating condition even though the level of the deterioration of the catalyst is the same (i.e., medium). On the other hand, the value of the ratio of the lengths LVOS/LVOM are maintained substantially the same in the stable operating condition and the transient operating condition, since both the length LVOS and LVOM equally increase when the operating condition becomes stable.
Though the above explanation is given for the case in which the downstream O.sub.2 sensor is not deteriorated, the same phenomena occur also in the case of the combination of the deteriorated downstream O.sub.2 sensor and the catalyst with a medium level of deterioration.
Therefore, during the stable operation of the engine, a problem occurs in that the catalyst with a medium level of deterioration is detected as being deteriorated in a similar manner as the catalyst with a high level of deterioration.